In today's electronics designs, power control includes voltage or current regulation. One very popular example that requires constant current control is the light emitting diode (LED) application. As a lighting source, the LED unit is required to work with wide range brightness, which is proportional with the forward current passing through the LED unit. Therefore, the LED current needs to be tightly regulated throughout a wide range of current changes.
However, while it is relatively easy to meet accuracy requirement at full current, it is challenging to achieve high accuracy at a low current without a large voltage drop in the current control device connected in series with the LED. In certain applications such as the 1-cell Li ion powered devices, the driving voltage, which is at the battery voltage, can be dropped to merely 100 mV above the backlighting LED voltage, leaving very low voltage “headroom” for the constant current control. This makes it difficult to directly drive the LED without stepping up the input voltage.
The predominant solution today to drive LED backlight with 1-cell Li ion is to step up the input voltage to ensure enough voltage headroom for the current control circuitry. There are two types of current control topologies, current source and current sink, depending on the location of the current regulation circuitry. “Current source” refers to high side current control while “current sink” refers to low side current control. In this document, current sink is used as an example for circuit comparison and implementation of proposed circuit. Similar concept can apply to current source topology as well.
FIG. 1 is a schematic diagram illustrating a typical current sink circuit used to control white LED current according to the prior art. The circuit is coupled between a voltage source represented by node 11 and node 12. The current source 13 and resistor 15 are coupled in series. The input terminal 17 of a non-inverting operational amplifier (NOA) 16 is coupled to the node 14 between the current source 13 and the resistor 15. The output terminal 19 of the NOA 16 is coupled to the gate terminal 20A of a field effect transistor (FET) 20. A light emitting diode (LED) 21 is coupled between the node 11 and the drain terminal 20B of the FET 20. A resistor 24 is coupled between the node 12 and the source terminal 20C of the FET 20. The feedback terminal 18 of the NOA 16 is coupled to the node 23. By adjusting the current source (I_ADJ) 13, the non-inverting input voltage (Vref1) of the NOA 16, i.e., the voltage at node 14, varies as a function of resistor (R1) 15. As the equation (1) indicated, once the circuit reaches a steady state, the voltage level of the non-inverting input (Vref1), i.e. the voltage at the node 14, and the inverting input (V_R2) of the NOA 16, i.e. the voltage at node 23, are very close:Vref1=V—R2+Vos,  (1)Wherein, Vos is the offset voltage of the NOA 16. Since V_R2 is directly proportional with the LED current, the LED current can be controlled and regulated by adjusting I_ADJ, as indicated in the equation (2):I_ADJ*R1=I_LED*R2+Vos,  (2)
The main drawback of this circuit is that when LED current is low, the voltage level of V_R2 and Vref1 are small. However, Vos for the NOA 16 remains constant and represents a much larger percentage error in a low LED current case, which causes big inaccuracy on the LED current. Therefore, this circuit is not suitable for wide range and high-accuracy requirement.
What is desired is a circuit to maintain high current accuracy over wide range of current while keeping the voltage “headroom” very low.